Many watercraft are designed to operate in a planing mode as well as in a displacement mode, particularly watercraft designed for recreational use. In a planing mode of operation, lift is derived from a downward deflection of water by the shape of the hull at relatively higher speeds than hulls operating in a displacement mode where lift is derived from the mass of water displaced by the hull. It is well-recognized that the force required to propel watercraft increases sharply with speed during displacement mode of operation and through a transition mode of operation where the watercraft speed causes the onset of planing. When planing is achieved, much of the wave drag is lost. The total drag force then decreases and increases only slightly with increased speed until reduction of the wetted surface decreases and then the wetted surface increases. Accordingly, to further reduce the wetted surface area, and drag at higher planing speeds, hulls have sometimes incorporated a step, generally formed as a substantially vertical surface following a planing surface. For a sailboard, these steps are generally near the back or stern of the board. As lift from planing increases with speed, the portion of the hull behind the step would be lifted clear of the water and was (or was assumed to be) substantially dry.
For small planing hulls such as those of sailboards, such steps and their location were a compromise of having the planing surface further forward, i.e. near the center of gravity when planing fast and a larger planing surface when the hull is starting to plane, i.e. when the board is moving slower and an attempt is being made to transition it from the displacement mode to a planing mode by exceeding a speed hereafter called the transition speed. Also, prior art steps on a sailboard hull had a flat or slightly rockered planing surface in front of the step.
From Bernoulli's theorem, since the water speed is greater at this step than it is after any expansion past the step, there is a vacuum that tends to form behind the step. Such a vacuum, of course, forms a drag force on this vertical surface in the sailboard hull or other watercraft hulls. This is why these prior art steps need to be ventilated as taught in U.S. Pat. No. 6,595,159 B2. However, it was not known how deep to make the step and there was little, if any, lift behind these prior art steps when the hull was transitioning from displacement to planing mode.
The main purpose of the steps, when there is sufficient wind or power, is to allow the hull to plane at a higher, more optimum attack angle thus reducing the wetted surface, decreasing the drag and increasing the hull speed. Generally for any planing hulls with a fixed center of gravity, the attack angle of the hull starts out above the optimum at transition speed then decreases to below the optimum as speed increases. Conversely the optimum angle is smallest at transition speed, due to some displacement lift, and then increases to 4 or 5 degrees as planing speed increases to about 30 mph where the displacement lift is essentially zero.
For sailboards, a sailboarder can change their position on the board or hull thus changing the center of gravity. However, almost all sailboarders lack the skill to achieve the optimum attack angle from transition speed to very fast planing speed on prior art sailboards with prior art steps, particularly since foot straps are provided in one location.
In many prior art steps, the step is across the whole planing surface as in the step on a flying boat, airplane pontoons and some boats. In other cases, the step may be formed by the end of a sponson. In either case, substantial drag at transition speeds is presented.
There are a number of additional problems with the prior art step.    1) At the transition speed, there is turbulence which forms behind the vertical portion of the step. This turbulence of course increases the wave drag in addition to the drag caused by the vacuum which tends to form behind a step.    2) Particularly for a step on the planing surface of a sailboard, there is a side portion of the step where the step angles back toward the rear of the board. The vortex that forms from the water coming off this section of the step can cause the surface behind the step to be wetted even when planing at higher speed. This then increases the wetted surface drag when it is desired to have this surface dry, i.e. free of contact with the water.
Ventilation of a fin as on a sailboard is when air is drawn in to the low pressure side of the fin. The resistance of a sail board fin to ventilation depends on the distance from the fin to the back of the board and the width of the planing surface to the side of the fin. In the prior art, the region behind the step is recessed deeper into the hull or board than the region next to the fin and the vortex which then forms can ventilate the planing surface back to the side of the step thus reducing the ventilation resistance of the fin.
Note: that NACA uses depth of step to denote the height of the step into the hull, see for instance NACA TN 1062 (1946), rocker is a term used in water craft, particularly in surf boards and sailboards, of slight positive 2nd derivative and camber is a term used in wings, hydrofoils or planing surfaces of negative 2nd derivative. The camber at the end of a planing surface either toward the rear or toward a chine is called cupping.